What Do All Those Numbers on My Glasses Prescription Actually Mean?
Don't worry—you don't need an optometry degree to read your prescription
Have you ever looked at your glasses prescription and thought:
What in the world do all these numbers mean?
You're definitely not alone.
You might see something that looks like:
OD: -2.00 -0.75 × 180
OS: -1.50 -1.00 × 010
…and unless you work in eye care, that probably means absolutely nothing to you.
The good news is that you really don't need to understand every little detail of your prescription. But I do think it's nice to understand your own eyes, so let's break it down without making this an optometry class.
First: OD and OS
This one's easy. OD = right eye, OS = left eye. They're abbreviations from Latin that somehow we've all agreed to continue using in eye care.
Why? … I have no idea.
You may also occasionally see OU, which simply means both eyes.
That's it.
One mystery solved.
What Do the + and – Numbers Mean?
The first number you'll usually see is called the sphere, or SPH.
This tells us about the basic focusing power of your eye.
A minus (-) prescription generally means you're nearsighted. Things up close may be easier to see, while things farther away become blurry. A plus (+) prescription generally means you're farsighted or need additional focusing power.
And the farther that number gets from zero, the more prescription power is needed.
For example:
-0.50 is a relatively small amount of nearsightedness.
-5.00 is considerably more.
That doesn't mean the person with -5.00 has “bad eyes.” I actually don't love describing prescriptions as good or bad. A prescription is simply a measurement of how much optical correction your eyes need to focus clearly.
You can have a high prescription and perfectly healthy eyes. You can also have no glasses prescription at all and still develop an eye-health problem. Prescription and eye health are two different things.
Okay…Then What Is Astigmatism?
This is probably the word that scares patients unnecessarily.
I'll tell someone, “You have a little astigmatism.” And sometimes I immediately get: “WAIT. Is that bad?!”
No! Astigmatism is extremely common.
It simply means your eye doesn't focus light exactly the same way in every direction. You've probably heard the classic explanation that an eye without astigmatism is shaped more like a basketball and an eye with astigmatism is shaped more like a football.
That's a helpful way to picture it, although real eyes are obviously a little more complicated than sporting equipment.
The important thing is: Astigmatism is just another part of your prescription that we can correct with glasses or contact lenses.
CYL = How Much Astigmatism
If your prescription has a number under CYL, or cylinder, that's the amount of astigmatism correction in your prescription.
You might see: -0.50 or -1.25 or -2.00. Generally, the farther that cylinder number is from zero, the more astigmatism correction is being prescribed.
Pretty simple.
But then you see another mysterious number next to it...
AXIS = Where the Astigmatism Goes
Your axis will be a number between 1 and 180.
For example: -1.00 × 180
The -1.00 tells us how much astigmatism correction there is.
The 180 tells us the orientation of that correction.
Think of it like giving someone directions. It's not enough to say: “Go 10 miles.” Okay...which direction? The cylinder tells us how much, and the axis tells us where.
One important thing patients sometimes misunderstand: A higher axis number does NOT mean you have more astigmatism. An axis of 180 isn't “worse” than an axis of 90. It's just a direction.
So Let's Read an Actual Prescription
Let's say your prescription is:
-1.00 -1.00 × 090
In very simple terms, that tells me:
-1.00 sphere → you have some nearsightedness.
-1.00 cylinder → you also have some astigmatism.
090 axis → tells me the orientation of that astigmatism correction.
And that's basically it! You do not need to calculate anything yourself. That's our job.
Why Does My New Prescription Look Completely Different From My Old One?
Here's another fun one. Sometimes patients bring me two prescriptions and say:
“These aren't even CLOSE. Which doctor is wrong?!” And occasionally…both prescriptions can actually represent the same correction.
There are two different ways eye doctors can write astigmatism prescriptions: plus cylinder and minus cylinder notation.
That means the sphere, cylinder, and axis numbers can all look different even though mathematically they describe the same optical prescription.
For example, these two prescriptions are equivalent:
-2.00 +1.00 × 180, and -1.00 -1.00 × 090
I promise there is math behind this; I also promise you probably don't want me to explain it. So if you're comparing two prescriptions and the numbers look wildly different, don't panic. Ask your eye doctor or optician to compare them for you.
What Does ADD Mean?
Welcome to the club! If you're in your 40s or beyond, another number may suddenly appear on your prescription: ADD.
The ADD, or addition, is extra magnifying power used for near vision.
As we get older, the natural lens inside the eye gradually loses some of its ability to change focus from far away to up close. This is called presbyopia, and it's a normal part of aging.
That ADD power can be incorporated into progressive lenses, bifocals, reading glasses, and other lens designs.
So if you see something like: ADD +1.50, it doesn't mean your distance prescription suddenly changed by +1.50. It's telling the person making your glasses how much additional power you need for near vision.
What About Prism?
You may occasionally see another section labeled PRISM.
Most glasses prescriptions don't have this.
Prism may be prescribed when we need to change how the eyes work together or help with certain eye-alignment or double-vision problems.
If you have prism in your prescription, I'd recommend having your doctor explain why it's there specifically for you, because there are many different reasons we may prescribe it.
And Your Glasses Prescription Is NOT Your Contact Lens Prescription
This is another really common misconception.
You generally can't take the numbers from your glasses prescription and simply order contact lenses with them.
Contact lenses sit directly on your eyes, while glasses sit a short distance in front of them. Contacts also have their own measurements and fitting considerations—including the shape and size of the lens and how it actually sits and moves on your eye.
For certain prescriptions, even the actual power may need to be different. That's why glasses prescriptions and contact lens prescriptions are separate prescriptions.
Please Bring Your Old Prescription!
If you're seeing a new eye doctor—or even coming back to see us after a while—bringing your previous glasses or contact lens prescription can be really helpful.
It gives us another piece of your history.
Maybe your prescription has barely changed for five years.
Maybe it's gradually becoming more nearsighted.
Maybe your astigmatism changed.
Maybe today's result looks surprisingly different from your previous one and we want to double-check it.
Your old prescription gives us something to compare with.
And if you don't have it? That's okay too. We can still examine your eyes and determine your prescription.
The Numbers Are Just Measurements
If there's one thing I want you to take away from all of this, it's this: Your glasses prescription isn't a grade for your eyes. A bigger number doesn't mean you failed your eye exam.
It's simply a set of measurements that tells us what lenses your eyes need to see clearly.
So the next time someone hands you a prescription that looks like:
OD -2.25 -0.75 × 175
OS -1.75 -1.25 × 005
hopefully instead of thinking... What is this secret optometrist code?!
You'll at least recognize: Right eye. Left eye. Nearsightedness. Astigmatism. Direction.
And that's honestly about as much optometry math as I think anyone should have to do outside of optometry school.
— Suzzane Li, OD, FAAO, ABCMO
Simpli Eye Care PLLC, Managing Optometrist